Ribosomal protein L4 and transcription factor NusA have separable roles in mediating terminating of transcription within the leader of the S10 operon of Escherichia coli.

Ribosomal protein L4 of Escherichia coli autogenously regulates both transcription and translation of the 11-gene S10 operon. Transcription regulation occurs by L4-stimulated premature termination at an attenuator hairpin in the S10 leader. This effect can be reproduced in vitro but depends on the addition of transcription factor NusA. We show that NusA is required to promote RNA polymerase pausing at the termination site; such paused transcription complexes are then stabilized further by r-protein L4. The L4 effect is observed even if the protein is added after the NusA-modified RNA polymerase has already reached the pause site. Genetically separable regions of the S10 leader are required for NusA and L4 action: The attenuator hairpin is sufficient for NusA-dependent pausing, but upstream elements are necessary for L4 to prolong the pause.